1. Technical Field
The present invention relates to a secondary battery, specifically, to a secondary battery having excellent stability and improved output characteristic and low temperature characteristic by including a cathode active material in which at least one of metals forming the cathode active material has a concentration gradient in an entire region from a central portion up to a surface portion; and a conductive material mixture in which carbon nanotube is mixed with carbon black at an appropriate ratio, the carbon black being a spherical nanoparticle.
2. Description of the Related Art
Nowadays, a lithium secondary battery has been widely used in portable mobile devices such as cell phones, and laptop computers. In addition, the lithium secondary battery has received attention as a medium- or large-sized battery system for power storage of renewable energy such as solar and wind power, and the like, as well as power sources of hybrid electric vehicles and intelligent robots.
The lithium secondary battery uses a principle in which electrical energy is generated by a change in chemical potential when lithium ions are inserted into and desorbed from a cathode and an anode.
The lithium secondary battery is manufactured by including materials capable of reversibly inserting and desorbing lithium ions as a cathode active material and an anode active material, and filling an organic electrolyte or a polymer electrolyte between the cathode and the anode.
In particular, a lithium composite metal compound has been used as the cathode active material of the lithium secondary battery. For example, composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (0<x<1), LiMnO2, LiFePO4, and the like, have been researched so far.
Among the examples, since LiCoO2 is significantly useful as the cathode active material for a secondary battery due to stable charge and discharge characteristics, excellent electron conductivity, high battery voltage, high stability, and flat discharge voltage characteristics, but has a unstable crystal structure due to lithium elimination during charging, which significantly deteriorates thermal characteristics.
In order to overcome the drawback, Korean Laid-Open Publication Nos. 2005-0083869, 2007-0097923 suggested a lithium transition metal oxide in which metal composition has a concentration gradient, as a cathode active material for a secondary battery. However, due to a large internal resistance, the lithium transition metal oxide has lower output characteristic than that of the existing cathode active materials.